backports: add missing returns on ww_mutex backpots

Two returns were missing on the ww_mutex backport, add these.
The enhancements are not supported on older kernels so just
bail out early.

Tested against next-20130802

1   2.6.25              [  OK  ]
2   2.6.26              [  OK  ]
3   2.6.27              [  OK  ]
4   2.6.28              [  OK  ]
5   2.6.29              [  OK  ]
6   2.6.30              [  OK  ]
7   2.6.31              [  OK  ]
8   2.6.32              [  OK  ]
9   2.6.33              [  OK  ]
10  2.6.34              [  OK  ]
11  2.6.35              [  OK  ]
12  2.6.36              [  OK  ]
13  2.6.37              [  OK  ]
14  2.6.38              [  OK  ]
15  2.6.39              [  OK  ]
16  3.0.79              [  OK  ]
17  3.1.10              [  OK  ]
18  3.10-rc1            [  OK  ]
19  3.2.45              [  OK  ]
20  3.3.8               [  OK  ]
21  3.4.46              [  OK  ]
22  3.5.7               [  OK  ]
23  3.6.11              [  OK  ]
24  3.7.10              [  OK  ]
25  3.8.13              [  OK  ]
26  3.9.3               [  OK  ]

real    34m47.935s
user    938m15.592s
sys     130m11.724s

Signed-off-by: Luis R. Rodriguez <mcgrof@do-not-panic.com>